Systems and methods for closed loop regeneration of gas dehydration units using liquid petroleum gas

ABSTRACT

Water-saturated desiccant in a dehydration unit, having previously been used to dehydrate natural gas, is regenerated in a closed loop process using liquid petroleum gas (LPG). LPG is pumped from a storage tank, vaporized and superheated. The superheated LPG gas enters the dehydration unit such that the hot gas passes over the desiccant thereby regenerating the desiccant. An overhead stream from the dehydration unit passes to a condenser where the temperature of the hot gas from the dehydration unit is dropped to form a fluid stream containing LPG, water and non-condensable gases. The fluid stream passes to a three phase separator for separating the fluid stream into a gas stream, a water stream, and a liquid stream containing LPG which is then returned to the storage tank for reuse in the closed loop process.

FIELD

The present disclosure relates to the field of natural gas dehydrationunits that utilize adsorption beds containing solid desiccant, whichincludes molecular sieve, alumina, and silica gel, and further relatesto systems and methods for regenerating the adsorption beds in suchnatural gas dehydration units.

BACKGROUND

In conventional natural gas conditioning, natural gas, having passedthrough an acid gas removal unit (AGRU) and dewpoint control, is oftendehydrated by passing the natural gas through a system of vessels orunits referred to as a dehydration unit containing adsorption beds madeup of particulate material, also referred to interchangeably herein assolid desiccant, molecular sieve or mole sieve. Such a system includesat least two vessels in which one of the vessels contains saturated molesieve that is in regeneration mode, while the other one or more vesselsare operated in dehydration mode. During dehydration mode, water andother contaminants are adsorbed onto the mole sieve material; and duringregeneration mode, they are desorbed from the mole sieve. Typically, theregeneration begins by passing hot dry natural gas, i.e., natural gashaving been dehydrated, over the saturated mole sieve. This requires alarge compressor to return hot dry natural gas to a location upstream ofthe dehydration unit or the AGRU.

Dehydration of natural gas is typically accomplished by flow of hot gasover zeolite-based molecular sieve adsorbent. Water in the gas ispreferentially adsorbed by the molecular sieve. Removal of water fromthe gas using molecular sieve dehydration is a vital process componentin any liquefied natural gas (LNG) plant to meet moisture contentspecifications (down to 0.1 ppmv). Natural gas can contain additionalcontaminants such as hydrogen sulfide, mercaptans, oxygen, carbondioxide, carbonyl sulfide, etc. that are partially co-adsorbed by themolecular sieve. During high pressure regeneration, system designproblems such as hydrocarbon and water refluxing can result in poorwater desorption (high residual water content within the mole sieve) andcorrosion. This can result in early moisture breakthrough and economiclosses associated with frequent mole sieve change-outs and lowdehydrator availability.

If the mole sieve bed is regenerated at high temperature and lowpressure, then the regeneration gas may be a slip stream of dry gas, LNGboil off gas, or any other suitable dry gases. If the regeneration isconducted at high pressure and large vessel diameters, then the vesselthickness and choice of materials will create additional heat load onthe regeneration system.

The regeneration gas contains contaminants such as oxygen that reactswith hydrogen, hydrogen sulfide and/or hydrocarbon (e.g. propane) athigh regeneration temperatures resulting in the formation of unwantedby-products such as sulfur, sulfur di-oxides, water and carbon dioxide.These by-products can build up in downstream units, or in the fuelsystem causing problems such as fouling, and off-specification products.Furthermore, the complete regeneration of molecular sieves is notachieved because of the contaminants present resulting in sub-optimalperformance of the dehydration unit. This may also be accompanied bydamage caused to the molecular sieve resulting in reduced operatinglife. One known solution is further purification of the regeneration gasby using additional adsorbents. However, such schemes are expensive andwill not always result in full contaminant removal of the regenerationgas.

There exists a need for a more efficient, more reliable and less costlymethod and system for regenerating saturated mole sieve in a natural gasdehydration unit.

SUMMARY

In one aspect, a system is provided for regenerating water saturatedmole sieve in a gas dehydration unit containing the water saturated molesieve used in a process for dehydrating a natural gas feed stream. Thesystem includes a storage tank for storing liquid propane and/or butane(also referred to as LPG); a pump for pumping the LPG from the storagetank; at least two heat exchangers in series for receiving andconverting the LPG to a hot (superheated) propane and/or butane gas; aregeneration gas inlet in the gas dehydration unit containing the watersaturated mole sieve to be regenerated for receiving the hot propaneand/or butane gas such that the hot propane and/or butane gas passesacross the water saturated mole sieve thereby regenerating the watersaturated mole sieve; a condenser in communication with a regenerationgas outlet in the gas dehydration unit for receiving an overhead streamcontaining the hot propane and/or butane gas from the gas dehydrationunit and dropping the temperature to form a fluid stream containing LPG,water and non-condensable gases; a three phase separator incommunication with the condenser for separating the fluid stream into agas stream, a water stream, and a liquid stream comprising LPG; and aline in communication with the three phase separator for returning theLPG to the storage tank.

In one aspect, a method is provided for regenerating water saturatedmole sieve in the gas dehydration unit containing the water saturatedmole sieve. LPG is pumped from the storage tank to the at least two heatexchangers in series where the LPG is converted to hot propane and/orbutane gas. The hot propane and/or butane gas is introduced into theregeneration gas inlet in the gas dehydration unit such that the hotpropane and/or butane gas passes across the water saturated mole sievethereby regenerating the mole sieve. Next, the hot propane and/or butanegas from the regeneration gas outlet in the gas dehydration unit ispassed to a condenser, and the temperature of the hot propane and/orbutane gas is dropped to form a fluid stream containing LPG. The fluidstream is separated into a gas stream, a water stream, and a liquidstream comprising LPG. The LPG is finally returned to the storage tank.

DESCRIPTION OF THE DRAWINGS

These and other objects, features and advantages of the presentinvention will become better understood with reference to the followingdescription, appended claims and accompanying drawings. The drawings arenot considered limiting of the scope of the appended claims. Theelements shown in the drawings are not necessarily to scale. Referencenumerals designate like or corresponding, but not necessarily identical,elements.

FIGS. 1-4 are schematic diagrams illustrating systems for regenerating avessel of a dehydration unit according to exemplary embodiments.

DETAILED DESCRIPTION

In one embodiment, referring to FIG. 1, a system 100 and its operationfor regenerating water saturated mole sieve in a gas dehydration unitused in a process for dehydrating a natural gas feed stream will now bedescribed. As shown, the system 100 includes a line 1 for feedingnatural gas from a natural gas source (not shown) to the gas dehydrationunit which consists of at least two adsorbent beds including vessel 2and vessel 6. Vessel 2 is shown in dehydration mode, such thatmoisture-containing natural gas enters at the top of the vessel anddehydrated natural gas exits at the bottom of the vessel. Dehydrated gasis collected in line 7 and can be filtered in filter 8. Filtereddehydrated natural gas can be sent for further gas processing throughline 56.

Vessel 6 is shown in regeneration mode. Vessel 6 containswater-saturated desiccant. Vessel 6 has a regeneration gas inlet 6 a atthe bottom thereof, and a regeneration gas outlet 6 b at the topthereof. In an embodiment not shown, the top and bottom could bereversed such that the regeneration gas inlet 6 a is at the top ofvessel 6, and the regeneration gas outlet 6 b is at the bottom thereof.

As is well-known, the direction of flow through the gas dehydration unitvessels 2 and 6 will depend on the mode in the cycle being carried outat any given time. For example, during absorption mode, also referred toas dehydration mode, the flow can be directed from top to bottom. Duringdepressurization mode, flow would then also be directed from top tobottom. Likewise during preheating mode, flow would be directed from topto bottom. During heating mode, also referred to as regeneration mode,the flow would be directed from bottom to top. During cooling mode, flowwould also be directed from bottom to top. During drainage mode, flowwould be directed from top to bottom. Finally, during pressurizationmode, in preparation for absorption mode, flow would be directed fromtop to bottom. In another embodiment, each of these directions could bereversed.

Optionally, a heater 55 can be added upstream of a switching valve 16 topreheat the vessel 6 (in regeneration mode) until the bed outlettemperature is 5° C. above the dew point temperature of the LPG. Anotheroption is to use superheated LPG in down flow direction using valve 56,in place of using hot natural gas line 9 and heater 55 to preheat thevessel 6 (in regeneration mode) and the bed. Once the bed temperaturehas reached a desired value the regeneration of the bed will switch toup flow direction.

Optionally, additional vessels (not shown) may be present as one ofordinary skill in the art would understand, both in adsorption andregeneration mode.

A storage tank 32 contains liquid propane and/or butane, also referredto interchangeably herein as liquid petroleum gas or LPG. LPG from thestorage tank 32 is pumped by pump 34 from the tank 32 through a line toa heater 30. In one embodiment, the heater 30 can be two heat exchangersin series. A flow controller 31 can optionally be included between thepump 34 and the heater 30. The heater 30 receives and converts the LPGto a hot propane gas and/or butane gas. By “hot propane gas and/orbutane gas” is meant that the propane and/or butane gas is superheated,i.e., significantly above its dew point temperature. The hot propane gasand/or butane gas passes through line 26 to the regeneration gas inlet 6a and into the vessel 6 where the hot gas passes across the watersaturated desiccant and thereby regenerates the saturated desiccant.

Upon leaving the regeneration gas outlet 6 b of the vessel 6, anoverhead stream including the propane and/or butane gas having moisturetherein passes through the line 21 to condenser 19. In the condenser 19,the temperature of the overhead stream is dropped to form a fluid streamcontaining LPG and water.

The fluid stream leaves the condenser 19 and enters a three phaseseparator 20 for separating the fluid stream into a gas stream, a waterstream, and a liquid stream including LPG. The gas stream exits theseparator 20 through line 37. A pressure controller 38 is used to dropthe stream pressure where it is recycled back as a low pressure feed gasstream. The water stream exits the separator 20 through line 62. Theliquid stream including LPG leaves the separator 20 through line 61. Inone embodiment, line 61 returns the LPG to the storage tank 32. In oneembodiment, the gas stream in line 37 returns to the feed stream in line1 through line 39.

In case of potential loss of LPG from the system, a flow controlleropens up a control valve 52 in line 51 to provide make-up LPG. Thesource of the make-up LPG (not shown) can be, for example, adepropanizer overhead stream or a debutanizer overhead stream. Controlvalve 50 in line 61 is controlled by a level controller on the separator20.

If excess LPG is produced, the line 53 will direct the additional LPG toa de-propanizer or de-butanizer system (not shown) using flow controlvalve 57 and line 58.

Immediately following the regeneration of the saturated desiccant, thevessel 6 is very hot and should be cooled prior to further use. In oneembodiment, to cool the vessel 6, the switching valve 27 is closed andswitching valve 28 is opened, the cool LPG from the storage tank 32 canbe introduced using line 29 into the lower opening 6 a in the vessel 6such that the LPG vaporizes in the vessel 6. During the initial coolingcycle, some of the liquid LPG will be converted to vapor which will exitthe regeneration bed 6 from the regeneration gas outlet 6 b. Thevaporized propane and/or butane will pass through the line 21 tocondenser 19 as previously described. The regeneration bed 6 is filledwith LPG until the level transmitter 17 indicates when the vessel 6 isfilled thus confirming that the regeneration vessel 6 is cooled.

Draining of the regeneration vessel 6 filled with LPG requires theswitching valve 22 and 28 to be closed. The switching valve 25 is openedthus draining the LPG from the regeneration vessel 6 from 6 a using line23 to deliver the LPG liquid to the condenser 19. The level indicator 18indicates when the vessel 6 is emptied of LPG. The level indicator 17indicates when the LPG level within the vessel 6 is considered full.

The regeneration vessel 6 is pressurized using by-pass inlet gas fromthe line 9; the control valve 15 is opened to pressurize theregeneration vessel 6. The switching valves 16 and 25 are opened and theswitching valve 22 is closed. When the regeneration vessel is equalizedin pressure using control valve 15 and when flow indicator 24 indicatesno liquid flow, the valves 25 and 16 are closed. A flow controller 14can optionally be included. The regeneration vessel 6 is then ready foruse in adsorption mode to remove water from the gas stream.

In one embodiment, referring to FIG. 2, an optional distillation column44 is located between the three phase separator 20 and the storage tank32. In this embodiment, line 61 transports the LPG to the distillationcolumn 44. A reboiler 45 can be used in conjunction with thedistillation column 44. Vapor is removed through an overhead line 59,and cooled in condenser 60. Liquid from the condenser 60 can beseparated in separator 41 into a light hydrocarbon gas stream (directedto line 54) and a water stream 42. A pressure controller 40 can beincluded in line 54. Liquid hydrocarbons can be pumped to thedistillation column 44 by pump 43. A heavies stream 47 is removed fromthe bottom of the distillation column 44. A stream 46 containing propaneand/or butane are cooled in a cross heat exchanger 48 and a liquidstream passes through line 49 to a cooler 35. Liquid propane and/orbutane are then returned to the LPG storage tank 32. Due to potentialloss of LPG from the system, the flow indicator 33 opens up controlvalve 52 using line 51 to make-up LPG. Control valve 50 in line 61 iscontrolled by a level controller on the separator 20.

If excess LPG is produced, the line 53 will direct the additional LPG toa de-propanizer or de-butanizer system (not shown) using flow controlvalve 57 and line 58.

In one embodiment, referring to FIG. 3, an optional dehydration unit,also referred to as 66/67, is located between the three phase separator20 and the LPG storage tank 32. The optional dehydration unit consistsof a plurality of adsorbent bed containing vessels 66 and 67. Theoptional dehydration unit further removes dissolved water from the hotpropane and/or butane gas. FIG. 3 shows vessel 66 in dehydration modeand vessel 67 in regeneration mode. As pictured, a line including valve70 directs hot propane and/or butane gas from line 26 to dehydrationunit 67 and a line directs gas from dehydration unit 67 to a locationjust upstream of condenser 19. Line 13 is used to cool the bed 67 byvaporizing liquid LPG in up flow direction after the heating cycle. Line51 can be used to deliver makeup LPG from, e.g., a du-butanizer (notshown) to vessel 66. LPG from vessel 66 is sent to storage tank 32.Switching valves 68 and 69 are used to direct flow of the propane and/orbutane to the vessel 66/67. Valve 71 can be provided to divert a sidestream of the propane and/or butane to a location in line 21 justupstream of condenser 19. Valve 72 can be provided downstream of controlvalve 52 and upstream of the vessel 66. Valve 73 can be provideddownstream of valve 72 for controlling a flow of liquid stream includingLPG from the separator 20 to the vessel 66. Valve 74 can be providedbetween the vessel 66 and storage tank 32. In another embodiment, eachof these directions could be reversed.

In one embodiment, referring to FIG. 4, an optional solid potassiumhydroxide treatment unit 64 is located between the three phase separator20 and the storage tank 32. The solid potassium hydroxide treatment unit64 receives the liquid stream containing LPG from the three phaseseparator 20 and removes hydrogen sulfide, carbonyl sulfide and/ormercaptans from the liquid stream. A waste potassium hydroxide stream 63can be removed from the solid potassium hydroxide treatment unit 64 andsent to waste storage (not shown) for proper disposal. A stream 61 fromthe solid potassium hydroxide treatment unit 64 can be treated inoptional coalescer 65 to remove treater waste carryovers including,e.g., potassium hydroxide, potassium sulfide, and potassium mercaptide.

If excess LPG is produced, the line 53 will direct the additional LPG toa de-propanizer or de-butanizer system (not shown) using flow controlvalve 57 and line 58.

Disclosed herein are various embodiments of closed loop regenerationsystems and methods. The embodiments disclosed herein are intended to beused in new gas plants or in retrofits of existing gas plants,particularly those having an inadequate regeneration system in whichregeneration gas flow rate and contamination issues are concerns. Theembodiments disclosed herein offer advantages for the heating step ofthe regeneration process and utilize existing regeneration facilitiesfor the polishing and cooldown steps.

The closed loop regeneration systems and methods disclosed herein reducethe water refluxing, inadequate regeneration gas flowrate andcontaminant problems of conventional systems by using a separateregeneration medium, such as LPG. The regeneration medium can bevaporized (superheated) for regeneration and then condensed, treated toremove compounds such as water and hydrogen sulfide that are desorbedfrom the molecular sieve bed, and then recycled. The treatment processconsists of a combination of a solid bed KOH treater and associatedcoalescer, stripping column, and a small liquid molecular sievedehydrator as a moisture guard bed. Based on the available LPG (propane,butane or a mixture of both) and facility infrastructure, one or more ofthese treating steps can be eliminated. The regeneration medium does notmix with the process gas keeping the adsorption and regeneration systemsindependent. This can result in better regeneration of the molecularsieves, complete removal of contaminants in the gas stream, and betteradsorption efficiency.

Advantages of the present closed loop regeneration systems and methodsover conventional systems include, but are not limited to the following.Regeneration at low pressure gives higher volumetric gas flow andresults in better heat transfer and gas distribution through themolecular sieve bed, thus reducing residual water content. Waterrefluxing in the molecular sieve bed during regeneration is reduced.Slow ramping using LPG vapor is easier to implement since thesuperheater hot oil flow rate can be more easily controlled than firedheaters. Cool down of the molecular sieve bed by vaporization of LPG isfaster. No compressor is needed to flow the regeneration medium since apump can be used as the primary power driver to ensure regeneration canbe performed at all times. No full flow recycle stream passes back to alocation upstream of the dehydration unit or the AGRU which results insmaller equipment sizes for the amine contactor, dehydrator andassociated equipment. Sulfur and oxygen free regeneration can beachieved which results in better dehydrator performance and increasedlongevity. Due to a cleaner regeneration medium, the closed loopregeneration process is more reliable and meets a more stringent 0.1 ppmmaximum water content specification in the dehydrator product gas. Theregeneration stripping column operating parameters can be adjusted forchanging contamination (water, light and heavy hydrocarbons, undesirablesulfur species, non-condensable) in order to provide the bestregeneration medium resulting in a lower overall cost. The closed loopregeneration systems and methods eliminate the release of hydrogensulfide gas during the molecular sieve bed change-out which reduces theoverall cycle and change-out times because it does not require floodingof the bed with water and subsequent connection to the flare system tovent the hydrogen sulfide gas. The closed loop regeneration process issafer than the current technologies as it does not expose plantpersonnel to hydrogen sulfide, has no water usage, and less wasteproducts are generated (other than waste KOH, if a KOH treater is used).Less exposure of cycling isolation valves to hydrogen sulfide occursduring regeneration in sour gas service. The closed loop regenerationsystems and methods using LPG shorten regeneration heating time andincreases standby time since LPG has a higher heat content (10-20%higher) as a carrier gas in comparison to natural gas. LPG regenerationreduces the occurrence of channeling within the dehydration bed sinceLPG gas allows for better fluid flow distribution within the dehydrationbed due to lower regeneration pressure. In the closed loop regenerationsystems and methods using LPG, LPG flowrate can be increased withoutaffecting plant feed capacity since regeneration is performed in aclosed loop system. This helps lower regeneration time.

It should be noted that only the components relevant to the disclosureare shown in the figures, and that many other components normally partof a gas dehydration system are not shown for simplicity.

For the purposes of this specification and appended claims, unlessotherwise indicated, all numbers expressing quantities, percentages orproportions, and other numerical values used in the specification andclaims are to be understood as being modified in all instances by theterm “about.” Accordingly, unless indicated to the contrary, thenumerical parameters set forth in the following specification andattached claims are approximations that can vary depending upon thedesired properties sought to be obtained by the present invention. It isnoted that, as used in this specification and the appended claims, thesingular forms “a,” “an,” and “the,” include plural references unlessexpressly and unequivocally limited to one referent.

Unless otherwise specified, the recitation of a genus of elements,materials or other components, from which an individual component ormixture of components can be selected, is intended to include allpossible sub-generic combinations of the listed components and mixturesthereof. Also, “comprise,” “include” and its variants, are intended tobe non-limiting, such that recitation of items in a list is not to theexclusion of other like items that may also be useful in the materials,compositions, methods and systems of this invention.

This written description uses examples to disclose the invention,including the best mode, and also to enable any person skilled in theart to make and use the invention. The patentable scope is defined bythe claims, and can include other examples that occur to those skilledin the art. Such other examples are intended to be within the scope ofthe claims if they have structural elements that do not differ from theliteral language of the claims, or if they include equivalent structuralelements with insubstantial differences from the literal languages ofthe claims. All citations referred herein are expressly incorporatedherein by reference.

From the above description, those skilled in the art will perceiveimprovements, changes and modifications, which are intended to becovered by the appended claims.

What is claimed is:
 1. A method for regenerating a gas dehydration unitcontaining saturated mole sieve used in a process for dehydrating anatural gas feed stream, comprising: a. providing a storage tank forstoring liquid propane and/or butane; b. pumping the liquid propaneand/or butane from the storage tank to at least two heat exchangers inseries; c. converting the liquid propane and/or butane to a hot propanegas and/or butane gas in the at least two heat exchangers; d.introducing the hot propane gas and/or butane gas into a regenerationgas inlet in the gas dehydration unit containing the saturated molesieve to be regenerated such that the hot propane gas and/or butane gaspasses across the saturated mole sieve thereby regenerating the molesieve; e. passing the hot propane gas and/or butane gas from aregeneration gas outlet in the gas dehydration unit to a condenser anddropping the temperature of the hot propane gas and/or butane gas toform a fluid stream containing liquid propane and/or butane; f.separating the fluid stream into a gas stream, a water stream, and aliquid stream comprising liquid propane and/or butane; and g. returningthe liquid propane and/or butane to the storage tank.
 2. The method ofclaim 1, further comprising introducing liquid propane and/or butanefrom the storage tank to an inlet in the gas dehydration unit such thatthe liquid propane and/or butane vaporizes therein and the gasdehydration unit is thereby cooled.
 3. The method of claim 2, furthercomprising introducing hot dry gas into an upper inlet of the gasdehydration unit thereby forcing liquid propane and/or butane downwardsand out through a lower outlet in the gas dehydration unit to dry themole sieve in the gas dehydration unit.
 4. The method of claim 3,further comprising directing the liquid propane and/or butane from thelower outlet in the gas dehydration unit to the condenser.